Ultrafast internal conversion in a low band gap polymer for photovoltaics: experimental and theoretical study

Fazzi, Daniele; Grancini, Giulia; Maiuri, Margherita; Brida, Daniele; Cerullo, Giulio; Lanzani, Guglielmo

doi:10.1039/c2cp23917e

Cited by 47 publications

(84 citation statements)

References 77 publications

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“…Indeed, a recent study of another donoracceptor copolymer (PCPDTBT) revealed that ultrafast IC occurs only at a particular twist angle of the backbone through a conical intersection, evidencing that IC can inuence molecular conformation. 62 For PCDTBT, we have also reported small differences in relaxation of the S 1 state directly excited or populated via IC, possibly caused by similar conformation effects. 3.2.3 CDTBT thin lm.…”

Section: Cdtbt In Solution (390 Nm Excitation)mentioning

confidence: 96%

Charge transfer relaxation in donor–acceptor type conjugated materials

et al. 2013

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The development of conjugated materials bearing electron-rich and electron-poor units along their backbone introduces new possibilities to control functionality for organic electronic applications through charge transfer character in ground and excited states. A thorough understanding of intramolecular dipoles and their evolution during excited state relaxation is necessary in order to fully exploit this opportunity. PCDTBT is an alternating donor-acceptor copolymer with high photovoltaic efficiency in bulk heterojunction solar cells. We use time-resolved femtosecond transient absorption spectroscopy in solution and in the solid state to study PCDTBT and the dTBT and CDTBT model compounds, fragments of the polymer chain. Higher solubility and slower relaxation make CDTBT particularly suitable to understand the mechanism of charge transfer relaxation in this class of materials.A progressive increase of charge transfer character from the initially moderately polar excited state is mainly driven by solvent reorganization and some torsional rearrangements. Similar relaxation in solid state CDTBT might ultimately lead to the formation of separate charges.

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Section: Cdtbt In Solution (390 Nm Excitation)mentioning

confidence: 96%

Charge transfer relaxation in donor–acceptor type conjugated materials

et al. 2013

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“…The peculiarities noted in ultrafast spectroscopic measurements of different DA copolymers [23,24,[26][27][28] are all explained within our generic theory. Two close-lying ground state absorptions [23,24] and two distinct transient PA bands, with strong overlap between PA 2 and PA T1 [20,26,27] (ii) The short lifetimes of the triplets generated through photoexcitation are to be expected.…”

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confidence: 87%

“…Grancini et al determined from ultrafast dynamics studies that the broad LE band in PCPDT-BT (the Supplemental Material [20] for the structures of this and other DA copolymers) is composed of two distinct absorptions [23,24] centered at 725 and 650 nm. TD-DFT calculations assign these to the S 0 → S 1 and S 0 → S 2 excitations, with, however, the oscillator strength of the second transition smaller by more than an order of magnitude [24]. Two transitions underlying the LE bands in copolymers with CPDT as the donor have been postulated also by Tautz et al [25].…”

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confidence: 99%

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

et al. 2015

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We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.PACS numbers: 78.66. Qn, 71.20.Rv, 71.35.Cc,The primary photophysical process in polymer solar cells is photoinduced charge transfer, whereby optical excitation at the junction between a donor conjugated polymer and acceptor molecules creates a charge transfer (CT) exciton whose dissociation leads to charge carriers. The donor polymeric materials used to be homopolymers such as polythiophene which absorb in the visible range of the solar spectrum [1]. Homopolymers have recently been replaced by block copolymers whose repeat units consist of alternating donor (D) and acceptor (A) moieties [2][3][4][5][6][7][8][9][10][11]. This architecture reduces the optical gap drastically, and the DA copolymers absorb in the near infrared, where the largest fraction of the photons emitted by the Sun lie. The power conversion efficiencies (PCEs) of organic solar cells with DA copolymers as donor materials have exceeded 10% [11], and there is strong interest in the development of structure-property correlations that will facilitate further enhancement of the PCE. Clearly, this requires precise understanding of the nature of the primary photoexcitations of DA copolymers.Existing electronic structure calculations of DA copolymers are primarily based on the density functional theory (DFT) approach or its time-dependent version (TD-DFT) [12][13][14][15][16][17][18]. The motivations behind these calculations have largely been to understand the localized versus delocalized character of the excited state reached by ground state absorption. Experimentally, DA copolymers exhibit a broad low energy (LE) absorption band at ∼ 700 − 800 nm and a higher energy (HE) absorption band at ∼ 400 − 450 nm [2][3][4]. There is agreement between the computational studies that the LE band is due to CT from D to A, and the HE band is a higher π-π * excitation.Recent optical studies indicate that the above simple characterization of the LE band might be incomplete, and as in the homopolymers [19], electron correlations play a stronger role in the photophysics of the DA copolymers than envisaged within DFT approaches. Grancini et al. determined from ultrafast dynamics studies that the broad LE band in PCPDT-BT (the Supplemental Material [20] for the structures of this and other DA copolymers) is c...

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“…This was studied in detail for another D-A copolymer by Lanzani et al and is strongly intricate with torsional relaxation. 136 Exciting at different positions of the rst absorption band in PCDTBT might be a better way to study the wavelength effect on ultrafast delocalization. Finally, the investigation by Banerji et al revealed that not only are the steady-state spectra of PCDTBT very similar in lm and solution, but also the emission dynamics and anisotropy during the rst 10 ps (Fig.…”

Section: Ultrafast Excited-state Processesmentioning

confidence: 99%

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

2013

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In this feature article, we review and examine evidence that the primary photoexcited species in conjugated polymers is considerably delocalized. Localization occurs via a series of complex relaxation mechanisms on the <200 femtosecond time scale. We show that short-lived delocalization in the neutral excited state and charge separated state of bulk heterojunction blends might play an essential role in ensuring efficient formation of free charge carriers for photovoltaic applications. Finally, the additional parameter of intramolecular charge transfer character in the excited state of more recently developed donor-acceptor copolymers is discussed. Both delocalization and partial charge transfer reduce the binding of the electron and hole in photoexcited organic semiconductors and can help to overcome the bottleneck to macroscopic current generation in polymer solar cells.

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Ultrafast internal conversion in a low band gap polymer for photovoltaics: experimental and theoretical study

Cited by 47 publications

References 77 publications

Charge transfer relaxation in donor–acceptor type conjugated materials

Charge transfer relaxation in donor–acceptor type conjugated materials

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

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